msm_fb: Update backlight level on resume after first pan display

Use shared workqueue to delay the update of backlight after
a first valid frame is displayed on the panel. This avoids any
transients being displayed during resume.

CRs-fixed: 361002
Change-Id: Ia2c2e39aae62e9ddfebe3a26fb4dbc969d0892c2
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>